Iran has warned of "harsh retaliation" following a US airstrike that killed Tehran's top general.
Supreme Leader Ayatollah Ali Khamenei described Gen Qassam Soleimani the "international face of resistance". State TV reported that nine others died in the attack.
Iran's Foreign Minister Mohammad Javad Zarif called the assassination "an act of state terrorism and violation of Iraq's sovereignty".
Gen Soleimani, the head of Iran’s elite Quds Force, was targeted because he “was actively developing plans to attack American diplomats and service members in Iraq and throughout the region”, the US Defence Department said.
US President Donald Trump - who had earlier in the night tweeted an image of an American flag - said the general was responsible for many deaths abroad and in Iran.
He said that he "should have been taken out many years ago".
U.S. Secretary of State Mike Pompeo says the action was taken to disrupt an "imminent" attack orchestrated by him.
Pompeo said Soleimani "was actively plotting in the region to take actions, the big action as he described it, that would have put dozens if not hundreds of American lives at risk".
He added: "We know it was imminent. This was an intelligence-based assessment that drove our decision-making process."
The secretary of state did not say where or when the attack had been expected to take place.
Foreign Secretary Dominic Raab has urged "all parties to de-escalate" after a US air strike killed Iran's top general, adding: "Further conflict is in none of our interests."
Labour leader Jeremy Corbyn urged Boris Johnson to stand up to the "belligerent actions and rhetoric" from the US.
In a statement, Mr Corbyn said: "The UK Government should urge restraint on the part of both Iran and the US, and stand up to the belligerent actions and rhetoric coming from the United States.
"All countries in the region and beyond should seek to ratchet down the tensions to avoid deepening conflict, which can only bring further misery to the region, 17 years on from the disastrous invasion of Iraq."
An adviser to Iran’s President Hassan Rouhani quickly warned US President Donald Trump of retaliation from Tehran.
“Trump through his gamble has dragged the US into the most dangerous situation in the region,” Hessameddin Ashena wrote on the social media app Telegram.
“Whoever put his foot beyond the red line should be ready to face its consequences.”
Israel has long outlined Iran as it's main enemy in the increasingly hostile Middle East, a stance reinforced by air strikes against Tehran's interest in Syria, Lebanon and Iraq.
Mr Netanyhu stood by his Western ally during his Friday morning statement, stating Israel "stands with the United States in its just struggle for peace, security and self-defense."
The airport strike also killed Abu Mahdi al-Muhandis, deputy commander of Iran-backed militias in Iraq known as the Popular Mobilisation Forces, and five others including the PMF’s airport protocol officer, Mohammed Reda, Iraqi officials said.
Mr Trump was holidaying on his estate in Palm Beach, Florida, but sent out a tweet of an American flag following the airstrike.
Fears oil prices could rise after attack
On Friday oil prices spiked amid worrying that tensions in the Middle East could cut off one of the world’s most important energy supply routes.
Traders sent the price of Brent crude, the international standard, soaring 3.6% to 68.75 dollars per barrel.
Around one fifth of the world’s oil supply passes through the Strait of Hormuz, a narrow choke point between Iran and the Arabian Peninsula.
It is a vital point for international energy markets, and markets can react to even the slightest escalation in the region.
Luke Bosdet, the AA’s fuel price spokesman, said that drivers could pay more at the pumps if the oil price increase is here to stay.
“As seen in May, oil sustained at above 70 dollars a barrel has the potential to push the price of petrol to around the 130p-a-litre mark,” he said.
Attack comes as US president faces impeachment amid rising Middle East tensions
The dramatic attack comes at the start of a year in which Mr Trump faces both a Senate trial following his impeachment by the US House of Representatives, and a re-election campaign. It marks a potential turning point in the Middle East and represents a drastic change for American policy toward Iran after months of tensions.
Tehran shot down a US military surveillance drone and seized oil tankers. The US also blames Iran for a series of attacks targeting tankers, as well as a September assault on Saudi Arabia’s oil industry that temporarily halved its production.
The tensions take root in Mr Trump’s decision in May 2018 to withdraw the US from Iran’s nuclear deal with world powers, struck under his predecessor, Barack Obama.
Soleimani was the target of Friday’s US attack, which was conducted by an armed American drone, according to a US official. His vehicle was struck on an access road near the Baghdad airport.
A senior Iraqi security official said the airstrike took place near the cargo area after Soleimani left his plane to be greeted by al-Muhandis and others. The official said the plane had arrived from either Lebanon or Syria.
Two officials from the PMF, who spoke on condition of anonymity, said Suleimani’s body was torn to pieces in the attack, while they did not find the body of al-Muhandis. A senior politician said Soleimani’s body was identified by the ring he wore.
It’s unclear what legal authority the US relied on to carry out the attack. American presidents claim broad authority to act without congressional approval when US personnel or interests are facing an imminent threat. The Pentagon did not provide evidence to back up its assertion that Soleimani was planning new attacks against Americans.
Democratic Connecticut Sen. Richard Blumenthal said Mr Trump owed a full explanation to Congress and the American people.
“The present authorisations for use of military force in no way cover starting a possible new war. This step could bring the most consequential military confrontation in decades,” Blumenthal said.
But Trump allies were quick to praise the action.
“To the Iranian government: if you want more, you will get more,” tweeted South Carolina Sen. Lindsey Graham.
For Iran, the killing represents more than just the loss of a battlefield commander, but a cultural icon who represented national pride and resilience while facing US sanctions.
While careful to avoid involving himself in politics, Soleimani’s profile rose sharply as US and Israeli officials blamed him for Iranian proxy attacks abroad.
While Iran’s conventional military has suffered under 40 years of American sanctions, the Guard has built up a ballistic missile program. It also can strike asymmetrically in the region through forces like Lebanon’s Hezbollah and Yemen’s Houthi rebels. The US has long blamed Iran for car bombings and kidnappings it never claimed.
As the head of the Quds, or Jersualem Force of Iran’s paramilitary Revolutionary Guard, Soleimani led all of its expeditionary forces and frequently shuttled between Iraq, Lebanon and Syria.
US officials say the Guard under Soleimani taught Iraqi militants how to manufacture and use especially deadly roadside bombs against US troops after the invasion of Iraq. Iran has denied that. Soleimani himself remained popular among many Iranians, who saw him as a selfless hero fighting Iran’s enemies abroad.